Circuit board, particulary for a power-electronic module, comprising an electrically-conductive substrate

ABSTRACT

The invention relates to a circuit board, particularly for a power-electronic module, comprising an electrically-conductive substrate which consists, at least partially and preferably entirely, of aluminium and/or an aluminium alloy. On at least one surface of the electrically-conductive substrate, at least one conductor surface is arranged in the form of an electrically-conductive layer applied preferably using a printing method and more preferably using a screen-printing method, said conductor surface being in direct electrical contact with the electrically-conductive substrate.

The invention concerns a printed circuit board, in particular for apower electronic module, comprising an electrically conductivesubstrate, wherein the substrate at least partially and preferablycompletely comprises aluminum and/or an aluminum alloy. The inventionfurther concerns a power electronic module including at least oneprinted circuit board and a method of producing a printed circuit board.

The material aluminum is of ever increasing significance in particularin the field of power electronics. Due to its comparatively low weightand the low costs aluminum is frequently used as a cooling body forelectronic components (for example LEDs, IGBTs or MOSFETs) in powerelectronic modules or directly as current-carrying conductors, inparticular as a current or bus bar. For those purposes of use aluminumhas a very high level of thermal conductivity and also a very high levelof electrical conductivity.

In the field of power electronics an insulated metal substrate (referredto for brevity as IMS) is frequently used as the substrate, whichincludes a core of aluminum and which is encased by an electricallyinsulating or dielectric layer. In that case the aluminum core is usedexclusively for improved heat conduction. The conductor tracksthemselves are arranged on the insulating layer and are not electricallycontacted with the aluminum core.

The object of the invention is to provide a printed circuit board of thegeneral kind set forth, on which electronic components can be arrangedto be electrically contactable with a substrate of the printed circuitboard. In particular the invention seeks to provide that an electroniccomponent can be soldered to a substrate of a printed circuit board,that predominantly comprises aluminum and/or aluminum alloy, in order tobe able to make electrical contacting of the electronic component withthe substrate.

According to the invention that object is attained by the features ofclaim 1. Advantageous configurations of the invention are recited in theappendant claims.

According to the invention it is therefore provided that arranged on atleast one surface of the electrically conductive substrate is at leastone conductor surface in the form of an electrically conductive layerapplied preferably by a printing process and particularly preferably bya screen printing process, wherein the conductor surface is directlyelectrically contacted with the electrically conductive substrate.

An aim of the invention is to achieve direct electrical contacting ofconductor tracks or conductor surfaces arranged on the substrate withthe substrate itself and to use the substrate as an electricallyconductor. In the case of the proposed printed circuit board anelectrically conductive conductor surface which can substantiallycomprise copper and which can be of a thickness of between 25 μm and 125μm, preferably between 90 μm and 110 μm, is arranged directly on asurface of the electrically conductive substrate. It is thereforepossible to dispense with an insulating layer arranged between thesubstrate and the conductor surface. That makes it possible on the onehand to achieve a simplified structure for the printed circuit board,whereby a printed circuit board can also be produced at lower cost. Onthe other hand in that way the substrate, in addition to its function asa heat-dissipating device, can also be used as the current-carrying partof the printed circuit board. That is advantageous in particular inrelation to power electronic modules and the high electrical currentsoccurring therein.

According to a particularly preferred embodiment it can be provided thatthe at least one surface of the electrically conductive substrate issubstantially flat. That makes it possible to substantially simplify theprocess for producing a printed circuit board. Thus for exampleconventional aluminum plates of a thickness of between about 1 mm and 3mm can be easily cut, sawn or stamped out, according to the respectiverequirements, without the surfaces of the aluminum plates having to beespecially treated.

In a preferred embodiment of the invention it can be provided thatarranged on the at least one surface of the electrically conductivesubstrate is at least one insulator surface in the form of a dielectriclayer preferably applied by a printing process and particularlypreferably by a screen printing process. In that respect the at leastone insulator surface can at least partially adjoin the at least oneconductor surface, and can preferably surround the at least oneconductor surface.

To prevent flash-over between current- or voltage-carrying parts and arelated short-circuit they must be arranged at a given spacing relativeto each other. For example the spacing or the air gap between twovoltage-carrying parts with a voltage difference of 400 V should be atleast 4 mm in accordance with the Standard DIN EN 60664-1 VDE 0110-1. Byvirtue of the insulator surface the spacing relative to othercurrent-carrying parts, for example other printed circuit boards withina power electronic module, with the same dielectric strength, can bereduced, for example to below 1 mm.

In that way it is possible to reduce structural sizes of powerelectronic modules which include at least one proposed printed circuitboard.

The thickness of the insulator surface can be selected in accordancewith the respective flash-over voltage to be rated. In the case of anaverage flash-over voltage of the dielectric layer of 800 V per 25 μm athickness of 100 μm for the insulator surface is usually sufficient. Ingeneral the thickness of the dielectric layer can be selected independence on the flash-over voltage of an IGBT used and arranged forexample between two printed circuit boards and for example can be soselected for high-voltage applications that a flash-over voltage ofbetween about 600 V and about 1700 V is achieved.

In general the insulator surface can also serve as a solder covering forthe at least one conductor surface. Therefore the at least one insulatorsurface can be applied to the substrate in such a pattern so that the atleast one conductor surface or a plurality of conductor surfaces aresurrounded or enclosed by the insulator surface.

To produce the at least one insulator surface on the substrate it can beprovided that a dielectric layer is applied at least region-wise to theat least one surface of the substrate. In that case a dielectricthick-layer paste can be applied by a printing process, preferably by ascreen printing process. The thick-layer paste can be dried attemperatures below about 200° C. for about 10 min or sintered directlyin a firing furnace.

Firing or sintering of the thick-layer paste can be effected in an airatmosphere at temperatures of between about 540° C. and about 640° C. ina firing furnace. It is also possible for the thick-layer paste to befired at temperatures below 540° C. but that can have a detrimentaleffect on adhesion of the thick-layer paste to the substrate. When thethick-layer paste is fired at over 640° C. the substrate can begin tosoften as the melting point of aluminum is at about 660° C.

To achieve advantageous adhesion of the thick-layer paste to thesubstrate glass constituents of the thick-layer paste can include atleast one alkali metal oxide, for example lithium oxide, sodium oxide orpotassium oxide. As a result the glass constituents already melt attemperatures below the melting point of aluminum. In addition, thecoefficient of expansion of the thick-layer paste can be increasedand/or or adapted to the coefficient of expansion of aluminum, by thepresence of alkali metal oxides.

A proposed printed circuit board is particularly suitable for being usedin compact power electronic modules, for example in high-currentmulti-phase power bridges or inverters. Such power electronic modulesfrequently employ electronic switches or transistors in the form ofbipolar transistors with insulated gate electrodes (referred as“insulated-gate bipolar transistors” or for brevity IGBT). For theconnection of such a gate electrode it can be provided that at least oneconnecting surface in the form of an electrically conductive layer isdisposed on the at least one insulator surface.

The connecting surface can be subsequently connected to the gateterminal of an IGBT, for example by soldering.

Protection is also claimed for a power electronic module as set forth inclaim 12. Advantageous configurations are recited in the claims appendedthereto.

A proposed printed circuit board can be part of a power electronicmodule, for example of an inverter. Such inverters are used inter aliain hybrid or fully electric drive trains in the automobile sector inorder to convert the dc voltage of a dc voltage source (for example abattery) into a 3-phase ac voltage for a three-phase motor. The inverteritself can in that case include six electronic switches (for exampleIGBTs) and free-wheeling diodes corresponding thereto. In that case, bysuitable actuation of the gate terminals of the IGBTs a dc voltageconnected to the inverter, for example in the range of between about 300V and 1200 V, can be converted in known manner into threephase-displaced ac voltages and fed to a three-phase motor.

According to a particularly preferred embodiment a proposed powerelectronic module can include a first printed circuit board, a secondprinted circuit board and three third printed circuit boards.

For the first printed circuit board it can be provided that a pluralityof conductor surfaces, preferably six conductor surfaces, are arrangedon the at least one surface of the electrically conductive substrate,wherein preferably the conductor surfaces are surrounded by an insulatorsurface. For example three IGBTs and three free-wheeling diodescorresponding thereto can be mounted to the conductor surfaces, forexample by soldering. The first printed circuit board can be for examplein the form of a negatively poled current bar of an inverter, that canbe connected to the negative pole of a dc voltage source.

For the second printed circuit board it can additionally be providedthat a plurality of connecting surfaces, preferably three connectingsurfaces, are arranged on the insulator surface. In that way, inaddition to mounting three IGBTs and three free-wheeling diodescorresponding thereto to the conductor surfaces, the gate electrodes ofthe IGBTs can also be connected to the connecting surfaces, for exampleby soldering, and subsequently actuated. The second printed circuitboard can be for example in the form of a positively poled current barof an inverter, that can be connected to the positive pole of a dcvoltage source.

For each of the three third printed circuit boards it can be providedthat a plurality of conductor surfaces, preferably two conductorsurfaces, are arranged on a first surface of the electrically conductivesubstrate, and a plurality of conductor surfaces, preferably twoconductor surfaces, and at least one insulator surface, are arranged ona second surface of the electrically conductive substrate, wherein aconnecting surface is arranged on the at least one insulator surface.

Each of the three third printed circuit boards can in that case be inthe form of a phase connection of an inverter for a three-phase motor.The two conductor surfaces of the first surface of a third printedcircuit board can in that case be connected, by example by soldering, toa respective pair of electronic components—including an IGBT and afree-wheeling diode corresponding thereto—which can be arranged on thesecond printed circuit board. In that case the two conductor surfacesand the connecting surface of the second surface of a third printedcircuit board can be connected, for example by soldering, to arespective pair of electronic components—including an IGBT and afree-wheeling diode corresponding thereto—which can be arranged on thefirst printed circuit board. The connecting surface can serve in thatcase for respectively connecting the gate electrode of an IGBT.

It has proven to be particularly advantageous if the printed circuitboards are arranged substantially stacked, wherein three third printedcircuit boards are arranged, preferably in mutually juxtaposedrelationship, between the first printed circuit board and the secondprinted circuit board. That makes it possible to provide that a powerelectronic module is of a very compact structure.

In a particularly preferred embodiment it can be provided that the powerelectronic module is in the form of a high-current multi-phase powerbridge, wherein three transistors, preferably IGBTs and threefree-wheeling diodes are mounted, preferably soldered, on the at leastone surface of the first printed circuit board and/or the secondsurfaces of the three third printed circuit boards, and wherein threetransistors, preferably IGBTs and three free-wheeling diodes aremounted, preferably soldered, on the at least one surface of the secondprinted circuit board and/or the first surfaces of the three thirdprinted circuit boards.

The operation of soldering electronic components like for example IGBTsand free-wheeling diodes on a proposed printed circuit board canpreferably be effected by vapor phase soldering. In that way a unitarytemperature gradient can be achieved in the solder layers of a powerelectronic module. In the case of a stacked inverter a first solderlayer can be arranged between the first printed circuit board and thethree third printed circuit boards and a second solder layer can bearranged between the three third printed circuit boards and the secondprinted circuit board.

In general a conductor surface can be applied to the surface of asubstrate by various processes like for example by galvanic processes,plasma metal sprays or by plating (for example roll plating).

Protection is also claimed for a method of producing a printed circuitboard as set forth in claim 16.

A chemical property of aluminum is a thin oxide layer which is veryquickly formed in the air and which is formed by contact with oxygen inthe atmosphere as a consequence of an oxidation process at the surfaceof an aluminum body. That oxide layer admittedly affords on the one handprotection from corrosion, but on the other hand it causes difficultiesin joining aluminum to other materials by soldering, welding or otherknown joining techniques.

For producing a proposed printed circuit board, in particular forproducing the at least one conductor surface on the substrate, it cantherefore be provided that a conductor paste is applied at leastregion-wise to a surface of the substrate, in a first firing phase theconductor paste is exposed to a substantially continuously rising firingtemperature, wherein the firing temperature is increased to apredeterminable maximum firing temperature of less than about 660° C.,in a second firing phase the conductor paste is exposed substantially tothe predeterminable maximum firing temperature for a predeterminableperiod of time, in a cooling phase the conductor paste is cooled downand in a post-treatment phase a surface of the conductor paste ismechanically post-treated, preferably brushed.

The regions at which the conductor paste is applied and sintered inaccordance with the method steps provide for electrical contacting ofthe substrate instead of the oxidized surface of the substrate, thatprevails in that region. That electrically conductive layer which isachieved at least region-wise by the application and sintering of theconductor paste can be subsequently used for example for soldering anelectronic component or also for soldering a cooling body, wherein thecooling body itself can in turn comprise aluminum.

In that case the substrate can at least partially and preferablycompletely comprise an aluminum material with a proportion of aluminumthat is as high as possible. Preferably an aluminum material is used, ofthe quality EN AW-1050A or EN AW-1060A in accordance with EuropeanStandard EN 573, containing at least 99.5% by weight or 99.6% by weightrespectively. In spite of somewhat lower liquidus temperatures and alower level of thermal conductivity in comparison with theabove-mentioned substantially pure aluminum materials it is alsopossible to use aluminum alloys, for example aluminum alloys includingmanganese or magnesium like for example EN AW-3003 (AlMn1Cu), EN AW-3103(AlMn1), EN AW-5005 (AlMg1) or EN AW-5754 (AlMg3).

The described production method gives the possible option of selectivelymetallizing individual regions of the surface of an aluminum-basedsubstrate, wherein the metallized regions are directly connected in theform of sintered conductor paste to the substrate in bonded joining ofthe materials involved and that makes it possible to achieve a highlevel of electrical conductivity and a high level of thermalconductivity of conductor paste to substrate and vice-versa. In additionthe metallized regions represent solderable regions by which thesubstrate can be connected to further components in known fashion. Thusfor example individual electronic components can be soldered on to themetallized regions using conventional soldering agents like eutecticSn—Pb—, Sn—Ag—Cu— or Sn—Au-solders.

According to a particularly preferred embodiment it can be provided thatthe conductor paste is applied to the surface of the substrate by aprinting process, preferably by a screen printing process.

In that case conventional conductor pastes in the form of thick-layerpastes or sinter pastes can be used. Different degrees of heat expansionof conductor paste and substrate can be compensated by the porosity ofthick-layer pastes, whereby the reliability of the connection betweenthe conductor paste and the substrate can be increased, in particular inrelation to major cyclic thermal stresses as for example in theautomobile field.

The additive nature of the screen printing procedure with which layersare built up on a substrate means it is also possible, for metallizing asubstrate surface, to dispense with the use of exposure and etchingprocesses, which leads to cost advantages for the proposed process.

A thick-layer conductor paste usually includes at least a metal powderas an electrically conductive agent, an inorganic powder (for exampleglass frits) as bonding agent, as well organic binding and dissolvingagents. The organic binding and dissolving agents lead to a paste-likeconsistency with given rheological properties, which however are alsoinfluenced by the further constituents of the conductor paste.

In regard to the constituent of the electrically conductive metal powderit can preferably be provided that a conductor paste including a copperpowder is used. It will be appreciated however that it is also possibleto use a conductor paste including a silver and/or gold powder. The useof copper powder is however markedly less expensive in that respect.

In regard to the constituent of the inorganic powder it can preferablybe provided that a conductor paste including a glass from thePbO—B₂O₃—SiO₂ system and/or a glass including Bi₂O₃ is used. In thatway, during the sintering procedure in the proposed method, in spite ofthe comparatively low firing temperatures prevailing in that situation,it is possible to achieve very good adhesion of the conductor paste tothe substrate.

After a conductor paste is applied by printing, for example by a screenprinting process known in the state of the art, the conductor pasteremains substantially on the corresponding regions by virtue of itsrheological properties, without flowing to any extent worth mentioning.To be able to optimally prepare the conductor paste applied to thesurface of the substrate for the firing or sintering operation, it canpreferably be provided that the conductor paste is dried prior to thefirst firing phase in a drying phase at a temperature of between about80° C. and about 200° C., preferably between 100° C. and 150° C.,particularly preferably at a maximum of 130° C., preferably for a periodof between about 5 min and about 20 min. Due to that drying phase thesolvents present in the conductor paste are substantially completelydissipated. Known drying methods like for example infrared or hot airdrying are preferred in that case. Due to the drying process and thedissipation linked thereto of the solvents in the conductor paste theconductor paste experiences a certain volume shrinkage. It is howeveralready possible to counteract that beforehand by application of theconductor paste in a correspondingly thicker layer.

Firing or sintering of the conductor paste in the first and/or secondfiring phase of the proposed method can preferably be effected in afiring furnace, wherein the firing temperature prevails in the firingfurnace. It will be appreciated that the drying phase and/or the coolingphase can also be effected in the firing furnace. Preferably in thatcase a firing furnace having a conveyor device can be used.

In dependence on the material combination used comprising substrate andconductor paste it is possible to apply a suitable firing profile. Aparticular variant provides that in the first firing phase the firingtemperature is increased at least temporarily by between about 40°C./min and about 60° C./min. It can further be provided that in thefirst firing phase the firing temperature is increased to a maximumfiring temperature of about 580° C., preferably about 565° C.,particularly preferably about 548° C.

Heating the conductor paste to above between about 400° C. and 450° C.provides that all organic constituents like for example organic bindingagents are substantially completely broken up and the inorganicconstituents (for example glass powder or glass frits) soften. Inaddition the metal powder sintering process begins at thosetemperatures. The softened glass constituents of the conductor pastesubsequently lead to good adhesion of the conductor paste on thesubstrate.

The maximum firing temperature is basically limited by the meltingtemperature of aluminum, which is at about 660° C. When using asilver-based conductor paste the maximum firing temperature ispreferably about 565° C. while when using a copper-based conductor pastethe maximum firing temperature is preferably about 548° C. Thosetemperatures result from the melting temperatures of possible eutecticaluminum-copper or aluminum-silver alloys which are involved in thatcase.

In regard to the respective maximum firing temperature glassconstituents suited to a conductor paste are to be selected, whosecorresponding glass transition temperature (T_(G)) or meltingtemperature (T_(S)) are adapted to that maximum firing temperature. Theglass transition temperature or melting temperature of the glassconstituent of the corresponding conductor paste should accordingly besuitably below the specified maximum firing temperature to ensureoptimum adhesion of the conductor paste to the substrate. In particularglasses from the PbO—B₂O₃—SiO₂ system or glasses including Bi₂O₃ aresuitable.

It has proven to be particularly advantageous if firing of the conductorpaste in the second firing phase is effected for between about 5 min andabout 30 min. It is possible in that way to achieve optimum adhesion ofthe conductor paste to the substrate. Basically, the longer the periodof time in the second firing phase (at maximum firing temperature), themore densely is the conductor paste sintered and thus has betterproperties for further processing (for example soldering and welding).With excessively long periods in the second firing phase however thetransit time in a typical firing furnace is correspondingly extended inlength, which can have an adverse effect on the overall through-put.

In a further advantageous embodiment it can be provided that thepredeterminable maximum firing temperature is kept substantiallyconstant in the second firing phase.

Preferably it can also be provided that the conductor paste is exposedto a protective gas atmosphere including nitrogen in the first firingphase and/or the second firing phase. The use or an inert gas orprotective gas means that it is possible to reduce or prevent oxidationof a copper contained for example in the conductor paste. That isadvantageous in particular at high temperatures. A protective gasatmosphere (for example nitrogen) is advantageous for burning in copperconductor track pastes to prevent oxidation of the conductor trackmaterial (depending on the firing phase there can be a residual oxygencontent of some ppm). The organic binders of such a material or of theconductor paste can be so conceived that they can be reduced in anitrogen atmosphere. In turn a conventional air atmosphere can beadvantageous for silver conductor track pastes because this does notinvolve any serious impairment of the conductor track surface due tooxidation. The organic binders used in that case can be oxidized by wayof the oxygen in the air.

In a preferred embodiment of the invention it can be provided that inthe cooling phase the firing temperature is reduced at least temporarilyby between about 20° C./min and about 40° C./min, preferably by about30° C./min. Preferably in that case cooling is effected to ambienttemperature. The slower the cooling operation, the correspondingly lessare the mechanical effects of the join between the conductor paste andthe substrate by virtue of different coefficients of thermal expansionof the materials used.

Due to the typical oxidation of the sintered conductor paste whichoccurs during the firing or sintering process due to the hightemperatures prevailing in that case it is provided that the surface ofthe conductor paste is suitably mechanically post-treated after thecooling step in order to facilitate further processing, for example forsubsequent soldering or welding processes.

According to a preferred embodiment it can be provided that theconductor paste is applied to the surface of the substrate in athickness of between about 10 μm and about 100 μm. It will beappreciated that it is also possible to apply conductor pastes to thesurface of the substrate in a thickness of less than 10 μm or conductorpastes in a thickness of more than 100 μm. It can also be provided thatthe proposed method is applied a plurality of time in succession toincrease the overall resulting thickness of the conductor paste.Preferably the at least one conductor surface of the proposed printedcircuit board, that can correspond to the sintered conductor paste, isof a thickness of between 25 μm and 125 μm, preferably between 90 μm and110 μm.

Further details and advantages of the present invention are described bymeans of the specific description hereinafter. In the drawing:

FIG. 1 shows a circuit diagram of a power electronic module in the formof an inverter,

FIG. 2 a shows a perspective view of a proposed printed circuit board,

FIG. 2 b shows the printed circuit board of FIG. 2 a with electroniccomponents arranged thereon,

FIG. 3 shows a further proposed printed circuit board with electroniccomponents arranged thereon,

FIG. 4 shows an embodiment of a proposed power electronic module duringassembly,

FIG. 5 shows a perspective view of a proposed power electronic module,

FIG. 6 shows a side view of a power electronic module as shown in FIG.5,

FIG. 7 a shows a sectional view along section line I-I in FIG. 5,

FIG. 7 b shows a detail view of FIG. 7 a,

FIG. 8 a shows a sectional view along section line II-II in FIG. 5,

FIG. 8 b shows a detail view of FIG. 8 a.

FIG. 1 shows a block circuit diagram of a power electronic module 2 inthe form of an inverter. The power electronic module 2 includes sixelectronic components 7 in the form of IGBTs U_(H), V_(H), W^(H), U_(L),V_(L), W_(L) and is connected to a dc voltage source 9, for example abattery. The gate terminals of the three highside transistors U_(H),V_(H), W_(H) and the three lowside transistors U_(L), V_(L), W_(L) areactuated in known manner by an electronic actuating means 10 so that thedc voltage of the dc voltage source 9 is converted by the powerelectronic module 2 into three phase-displaced ac voltages and fed to athree-phase motor 11. Each of the six IGBTs can additionally beconnected to a corresponding free-wheeling diode. For reasons of clarityof the drawing however those free-wheeling diodes are not shown in thisview.

FIG. 2 a shows a printed circuit board 1 b of a power electronic module2 in the form of an inverter as shown in FIG. 1. The printed circuitboard 1 b includes an electrically conductive substrate 3 in the form ofan aluminum plate, the surfaces 3 a, 3 b of which are substantiallyflat. The printed circuit board 1 b can be for example the positivecurrent bar of the inverter, that is to be connected to the positivepole of a dc voltage source 9 by means of a connecting element 12. Thesurface 3 a of the printed circuit board 1 b has three conductorsurfaces 4 a for IGBTs to be arranged thereon and three conductorsurfaces 4 b for free-wheeling diodes to be arranged thereon. Theconductor surfaces 4 a, 4 b are surrounded or bordered by an insulatorsurface 5. Both conductor surfaces 4 a, 4 b and also insulator surface 5can be applied to the surface 3 a of the substrate 3 in the form ofsuitable thick-layer pastes by means of screen printing and can be firedor sintered for example in a firing furnace. To be able to supply thegate terminals of the IGBTs with suitable control signals appropriateconnecting surfaces 6 are additionally arranged on the insulator surface5.

FIG. 2 b shows the printed circuit board 1 b of FIG. 2 a with IGBTs 7arranged on the conductor surfaces 4 a and with free-wheeling diodes 8arranged on the conductor surfaces 4 b. In this case the gate terminalsof the IGBTs 7 are connected to the connecting surfaces 6.

FIG. 3 shows a further printed circuit board 1 a similar to that of FIG.2 a—but without gate terminals or connecting surfaces 6—with IGBTs 7arranged on the conductor surfaces 4 a and free-wheeling diodes 8arranged on the conductor surfaces 4 b. In this case the electroniccomponents 7, 8 are soldered to the corresponding conductor surfaces 4a, 4 b for example by means of vapor phase soldering.

FIG. 4 shows an embodiment of a power electronic module 2 as shown inFIG. 1, wherein the power electronic module 2 includes a first printedcircuit board 1 a, a second printed circuit board 1 b and three thirdprinted circuit boards 1 c. In this case the first printed circuit board1 a corresponds to the printed circuit board 1 a shown in FIG. 3 and thesecond printed circuit board 1 b corresponds to the printed circuitboard 1 b shown in FIG. 2 a. The first printed circuit board 1 a can beconnected for example to the negative pole of a dc voltage source 9 bymeans of the connecting element 12 of the substrate 3 of the firstprinted circuit board 1 a, whereby the substrate 3 of the first printedcircuit board 1 a is in the form of a negative current bar. The secondprinted circuit board 1 b can be connected for example to the positivepole of a dc voltage source 9 by means of the connecting element 12 ofits substrate 3, whereby the substrate 3 of the second printed circuitboard 1 b is in the form of a positive current bar.

The three third printed circuit boards 1 c respectively include anelectrically conductive substrate 3 in the form of an aluminum plate,the surfaces 3 a, 3 b of which are substantially flat. A respectiveelectrically conductive conductor surface 4 a for an IGBT to beconnected thereto and an electrically conductive conductor surface 4 bfor a free-wheeling diode to be connected thereto are arranged at eachfirst surface 3 a of the substrate 3 of a third printed circuit board 1c. Arranged on each of the second surfaces 3 b of a substrate 3 of athird printed circuit board 1 c, in addition to the electricallyconductive conductor surfaces 4 a, 4 b, corresponding to the respectivefirst surface 3 a, is an insulator surface 5, on which an electricallyconductive connecting surface 6 is arranged for contacting a gateelectrode of an IGBT. Each substrate 3 of the three third printedcircuit boards 1 c has a connecting element 12, with which each of thethree third printed circuit boards 1 c is to be connected to a phase ofa three-phase motor 11.

As shown in FIG. 4, to assemble the power electronic module 2, theprinted circuit boards 1 a, 1 b, 1 c are stacked vertically one abovethe other in such a way that the three third printed circuit boards 1 care arranged in mutually juxtaposed relationship between the firstprinted circuit board 1 a and the second printed circuit board 1 b.Disposed between the first printed circuit board 1 a and the three thirdprinted circuit boards 1 c are three IGBTs 7 and three free-wheelingdiodes 8 which can be soldered to the respective conductor surfaces 4 a,4 b of the printed circuit boards 1 a, 1 c. Likewise in turn disposedbetween the three third printed circuit boards 1 c and the secondprinted circuit board 1 b are three IGBTs 7 and three free-wheelingdiodes 8 which can be soldered to the corresponding conductor surfaces 4a, 4 b of the first surfaces 3 a of the third printed circuit boards 1 cand the first surface 3 a of the second printed circuit board 1 b. Thegate terminals of the three IGBTs 7 between the first printed circuitboard 1 a and the three third printed circuit boards 1 c can becontacted by way of the connecting surfaces 6 on the second surfaces 3 bof the third printed circuit boards 1 c and the gate terminals of theIGBTs 7 between the three third printed circuit boards 1 c and thesecond printed circuit board 1 b can be contacted by way of theconnecting surfaces 6 of the first surface 3 a of the second printedcircuit board 1 b.

FIG. 5 shows a finished assembled power electronic module 2 as shown inFIG. 4 with the difference that an insulator surface 5 in the form of adielectric layer applied by a screen printing process is respectivelyarranged on both surfaces 3 a, 3 b of the three third printed circuitboards 1 c, wherein the respective insulator surface 5 of a surface 3 a,3 b surrounds the respective conductor surfaces 4 a, 4 b. Here inparticular the vertical stacking of the printed circuit boards 1 a, 1 b,1 c and the compact structure of the power electronic module 2 achievedin that way is also apparent.

FIG. 6 shows a side view of the power electronic module 2 of FIG. 5. Theconnecting elements 12 of the substrates 3 of the printed circuit boards1 a, 1 b, 1 c form in this case the connecting points to furthercomponents (see FIG. 1). In this arrangement the connecting element 12of the first printed circuit board 1 a can be connected to the negativepole of the dc voltage source 9 and the connecting element 12 of thesecond printed circuit board 1 b can be connected to the positive polethereof. The connecting elements 12 of the three third printed circuitboards 1 c can be connected to the corresponding phase connections of athree-phase motor 11.

FIG. 7 a shows a sectional view of the power electronic module 2 of FIG.5 along section line I-I and FIG. 7 b shows the region B marked with acircle in FIG. 7 a on an enlarged scale. The enlarged view in FIG. 7 bshows an IGBT 7 arranged between the first printed circuit board 1 a andone of the three third printed circuit boards 1 c of the powerelectronic module 2. In this case the IGBT 7 is soldered both on theconductor surface 4 a at the surface 3 a of the first printed circuitboard 1 a and also on the conductor surface 4 a at the surface 3 b ofthe third printed circuit board 1 c, for example by means of vapor phasesoldering. The solder used in that case is respectively indicated byreference 13. The conductor surfaces 4 a and also the conductor surfaces4 b (not visible here) on the surface 3 a of the first printed circuitboard 1 a and on the surface 3 b of the third printed circuit board 1 care surrounded by a dielectric insulator surface 5.

FIG. 8 shows a sectional view of the power electronic module 2 of FIG. 5along section line II-II and FIG. 8 b shows the region C marked with acircle in FIG. 8 a on an enlarged scale. In comparison with the detailview in FIG. 7 b it is possible to see in the enlarged view of FIG. 8 ban IGBT 7 arranged between the second printed circuit board 1 b and oneof the three third printed circuit boards 1 c of the power electronicmodule 2. The conductor surfaces 4 a, 4 b on the surface 3 a of thesecond printed circuit board 1 b and on the surface 3 a of the thirdprinted circuit board 1 c are surrounded by a dielectric insulatorsurface 5. The illustrated section along section line II-II in FIG. 5 isin the region of the gate terminal of the IGBT 7. To be able toelectrically actuate the gate of the IGBT 7 a connecting surface 6 inthe form of an electrically conductive layer is arranged on theinsulator surface 5 on the surface 3 a of the second printed circuitboard 1 b. Once again reference 13 denotes the respective solder usedfor soldering the IGBT 7 to the conductor surface 4 a of the thirdprinted circuit board 1 c and to the connecting surface 6 of the secondprinted circuit board 1 b.

In the case of a proposed power electronic module 2 with proposedprinted circuit boards 1 a, 1 b, 1 c electronic components 7, 8 can besoldered directly on to the substrates 3 of the printed circuit boards 1a, 1 b, 1 c by the provision of solderable electrically conductiveconductor surfaces 4 a, 4 b. As a result it is possible to dispense withother usual connecting procedures like for example wire bonding. Byvirtue of the additional provision of an insulator surface 5 it ispossible for the printed circuit boards 1 a, 1 b, 1 c to be arranged ina very compact fashion, for example to be stacked vertically, without inthat respect forfeiting dielectric strength. In the case of a stackedstructure therefore the spacing between two current- or voltage-carryingsubstrates 3 of printed circuit boards 1 a, 1 b, 1 c can be reduced tothe thickness of the electronic components 7, 8 (for example 250 μm of aconventional IGBT 7) and the thickness of the conductor surfaces 4 a, 4b (for example 100 μm). In the case of a power electronic module 2 inthe form of an inverter a reduced spacing between the highsidetransistors and the lowside transistors also makes it possible toachieve reduced inductance of the power electronic module 2 and thus toincrease the efficiency of the power electronic module 2.

In the production of a power electronic module it can be provided thatthe conductor surfaces 4 a, 4 b and the connecting surface 6 of aprinted circuit board 1 a, 1 b, 1 c are respectively jointly fired orsintered.

In a particularly preferred embodiment it can be provided that thewhole, preferably stacked, power electronic module 2 is finished in oneworking step insofar as the components 7, 8 (see FIG. 4) arrangedbetween the respective printed circuit boards 1 a, 1 b, 1 c are solderedin one working step to the respective conductor surfaces 4 a, 4 b andconnecting surfaces 6 (for example by vapor phase soldering). It will beappreciated that it can also be provided that the operation ofassembling the printed circuit boards 1 a, 1 b, 1 c is performed in aplurality of steps. For example the electronic components 7, 8 can berespectively soldered to the first printed circuit board 1 a and thesecond printed circuit board 1 b and in a further step the electroniccomponents 7, 8 can be soldered to the corresponding conductor surfaces4 a, 4 b and connecting surfaces 6 of the third printed circuit boards 1c. In that case the insulator surfaces 5 of the printed circuit boards 1a, 1 b, 1 c can also act as solder stop masks which hold the electroniccomponents 7, 8 in the desired positions during a soldering operation.

A solder paste to be arranged on the conductor surfaces 4 a, 4 b cangenerally also be used to better orient the substrates 3 of the printedcircuit boards 1 a, 1 b, 1 c with each other, insofar as for examplelayers of differing thickness of solder pastes are applied to theconductor surfaces 4 a, 4 b. In general it is also possible to useshaped solder pieces instead of solder paste.

Solders with different melting points can also be used for soldering.Thus for example an SnAgCu-solder with a liquidus temperature of about220° C. and a high-lead solder with a liquidus temperature of about 300°C. can be used. As a result for example firstly the electroniccomponents can be soldered with their first sides on the conductorsurfaces of a substrate with the high-lead solder and fixed there and ina further step the electronic components can be soldered with theirsecond sides using the SnAgCu solder on the conductor surfaces of afurther substrate. Accordingly therefore the components can be reliablyheld in position.

With the proposed printed circuit board it is generally possible toprovide a substrate which in addition to a heat dissipation functionalso takes over the function of an electrical conductor. By applyingelectrically conductive conductor surfaces and dielectric insulatorsurfaces to the substrate of a proposed printed circuit board on the onehand electronic components can be easily soldered to the substrate andthus electrically contacted while on the other hand it is possible toachieve compact structural configurations for power electronic modules,for example by vertical stacking. Spacings of voltage-carrying parts canbe reduced and thus the inductance of a power electronic module can bereduced by the insulator surfaces. In addition, direct, double-sidedcooling of a power electronic module can also be achieved by the use ofaluminum as the material for the substrates, and that permits highercurrent densities. By virtue of the provision of solder joins, it ispossible to dispense with other joining procedures like wire bonding,whereby the reliability of component connections can be increased. Whenusing thick-layer procedures for the production of conductor surfaces onthe substrate of a proposed printed circuit board it is also possiblefor the thermal resistance between an electronic component disposed onthe substrate and the substrate acting as a cooling body to be reducedby the direct assembly of components on the substrate, which is madepossible in that way. Due to the high porosity of a copper conductorpaste which is sintered at comparatively low temperatures it is alsopossible to reduce the mechanical stress in a solder layer between aconductor surface and an electronic component arranged thereon. Thatleads in particular to a higher temperature cycle resistance and anincreased service life.

1. A printed circuit board, in particular for a power electronic module,comprising an electrically conductive substrate, wherein the substrateat least partially and preferably completely comprises aluminum and/oran aluminum alloy, wherein arranged on at least one surface of theelectrically conductive substrate is at least one conductor surface inthe form of an electrically conductive layer applied preferably by aprinting process and particularly preferably by a screen printingprocess, wherein the conductor surface is directly electricallycontacted with the electrically conductive substrate.
 2. A printedcircuit board as set forth in claim 1, wherein the at least one surfaceof the electrically conductive substrate is substantially flat.
 3. Aprinted circuit board as set forth in claim 1, wherein the conductorsurface substantially comprises copper.
 4. A printed circuit board asset forth in claim 1, wherein the conductor surface includes a glassfrom the PbO—B₂O₃—SiO₂ system and/or a glass including Bi₂O₃.
 5. Aprinted circuit board as set forth in claim 1, wherein the conductorsurface is of a thickness of between 25 μm and 125 μm, preferablybetween 90 μm and 110 μm.
 6. A printed circuit board as set forth inclaim 1, wherein arranged on the at least one surface of theelectrically conductive substrate is at least one insulator surface inthe form of a dielectric layer preferably applied by a printing processand particularly preferably by a screen printing process.
 7. A printedcircuit board as set forth in claim 6, wherein the at least oneinsulator surface at least partially adjoins the at least one conductorsurface and preferably surrounds the at least one conductor surface. 8.A printed circuit board as set forth in claim 6, wherein at least oneconnecting surface in the form of an electrically conductive layer isarranged on the at least one insulator surface.
 9. A printed circuitboard as set forth in claim 1, wherein a plurality of conductorsurfaces, preferably six conductor surfaces, are arranged on the atleast one surface of the electrically conductive substrate, whereinpreferably the conductor surfaces are surrounded by an insulatorsurface.
 10. A printed circuit board as set forth in claim 9, wherein aplurality of connecting surfaces, preferably three connecting surfaces,are arranged on the insulator surface.
 11. A printed circuit board asset forth in claim 1, wherein a plurality of conductor surfaces,preferably two conductor surfaces, are arranged on a first surface ofthe electrically conductive substrate, and that a plurality of conductorsurfaces, preferably two conductor surfaces, and at least one insulatorsurface are arranged on a second surface of the electrically conductivesubstrate, wherein a connecting surface is arranged on the at least oneinsulator surface.
 12. A power electronic module comprising at least oneprinted circuit board as set forth in claim
 1. 13. A power electronicmodule comprising three printed circuit boards, each printed circuitboard being configured as set forth in claim
 11. 14. A power electronicmodule as set forth in claim 13, wherein the printed circuit boards arearranged substantially stacked, wherein the three printed circuit boardsare arranged, preferably in mutually juxtaposed relationship, betweenthe first printed circuit board and the second printed circuit board.15. A power electronic module as set forth in claim 13, wherein thepower electronic module is in the form of a high-current multi-phasepower bridge, wherein three transistors, preferably IGBTs and threefree-wheeling diodes are mounted, preferably soldered, on the at leastone surface of the first printed circuit board and/or the secondsurfaces of the three third printed circuit boards, and wherein threetransistors, preferably IGBTs and three free-wheeling diodes aremounted, preferably soldered, on the at least one surface of the secondprinted circuit board and/or the first surfaces of the three thirdprinted circuit boards.